epoch, unit of geologic time that is a subdivision of a period. The Pleistocene and Holocene epochs, for example, are divisions of the Quaternary period. Epoch is also used to describe a short length of geologic time during a special occurrence, such as the glacial epoch. See geology; Geologic Timescale (table).

Pliocene epoch, fifth epoch of the Cenozoic era of geologic time (see Geologic Timescale, table), from 5.1 to 2 million years ago. By the beginning of the Pliocene, the outlines of North America were almost the same as in recent time. Encroachments by the sea were limited to a narrow strip along the coasts of the Carolinas, Georgia, Florida, and the Gulf Coast states, and an embayment, smaller than that of the preceding Miocene epoch, in California. The Pliocene formations on the Atlantic coast are chiefly marine marls; on the Gulf they are nonmarine sediments resulting from erosion. In California they contain much volcanic ash and some are oil-bearing. The Pliocene formations of the western interior are small and scattered. In western interior North America and on the west coast, volcanic activity continued into the Pliocene from the Miocene. The close of the Pliocene was marked in North America by the Cascadian revolution, in the course of which the Sierra Nevada was elevated and tilted to the west. The Cascades, Rockies, Appalachians, and the Colorado plateau were uplifted, and there was activity in the mountains of Alaska and in the Great Basin ranges of Nevada and Utah. In Europe the Pliocene sea covered small parts of the northwest of the continent and a large area around the present Mediterranean; a number of volcanoes were active, among them Vesuvius and Etna. There was considerable mountain building, including the folding and thrusting of the Alps. The climate of the Pliocene was markedly cooler and drier than that of the Miocene and foreshadowed the glacial climates of the Pleistocene epoch. The life of the Pliocene was notable for its modern appearance; the Pliocene marked the climax, and perhaps the initial decline, of the supremacy of the mammals.

Pleistocene epoch, 6th epoch of the Cenozoic era of geologic time (see Geologic Timescale, table). According to a classification that considered its deposits to have been formed by the biblical great flood, the epoch was originally called the Quaternary. Analyses of the magnetic polarity in deep-sea sediment cores indicated that the Pleistocene began more than 1.8 million years ago—much earlier than had previously been suspected (see glacial periods). Since the interglacial periods of the Pleistocene were of longer duration than the time elapsed since the end of the Pleistocene 11,000 years ago, it is sometimes suggested that the Holocene, or Recent, epoch, which is occurring now, may be merely another such interglacial stage and that the glaciers may return at some future time.

An Ice Age

The Pleistocene is the best-known glacial period (Ice Age) of the earth's history. Its ice sheets at one time covered all of Antarctica, large parts of Europe, North America, and South America, and small areas in Asia. In North America they stretched over Greenland and Canada and over the United States as far south as a line drawn westward from Cape Cod through Long Island, New Jersey, and Pennsylvania, along the line of the Ohio and Missouri rivers to North Dakota, and through N Montana, Idaho, and Washington to the Pacific. The ice sheets of Europe radiated from Scandinavia and covered Finland, NW Russia, N Germany, and the British Isles. Glaciers distinct from the main sheets were formed in the Rockies and the Alps. In South America, Patagonia and the S Andes lay under an extension of the antarctic sheet, while in Asia the Caucasus, the Himalayas, and other mountain regions were glaciated.

The glaciation of the Pleistocene was not continuous but consisted of several glacial advances interrupted by interglacial stages, during which the ice retreated and a comparatively mild climate prevailed. In all probability there were actually only four glacial stages, the Iowan and Bradyan being included in the Wisconsin as one complex stage. Carbon-14 analysis of fossils shows that the last glacial period ended about 11,000 years ago.

Topographic and Climatic Changes during the Pleistocene

The characteristic formation laid down in the glacial stages of the Pleistocene, as in all glacial periods, is the drift. The interglacial stages were marked by the weathering of the till of the drift to form a sticky, heavy soil called the gumbotil and by the deposition of peat and loess. Peat is plentiful in the Aftonian, Yarmouth, and Sangamon interglacial stages in North America.

The Pleistocene glaciers made important alterations in the topography of the glaciated regions, leveling hilly sections to low, rolling plains, both by erosion and by deposition of drift, eroding hollows that later became lakes, and forcing rivers to cut new channels by filling their former beds. Among the characteristic surface features formed in the Pleistocene are the drumlin, kame, esker, and moraine. The retreat of the ice after the Wisconsin glacial stage was followed by the formation, at the edge of the melting glaciers, of lakes, such as the extinct Lake Agassiz and the Great Lakes. The further retreat of the ice led to the flooding by the Atlantic of the NE United States and SE Canada, which had been depressed below sea level by the weight of the ice. In the areas of North America not covered by ice, the Pleistocene was marked chiefly by erosion, with only very slight marine transgressions over the coast.

During the various glacial stages many areas not covered with ice, including the arid and semiarid parts of the W United States, had periods of increased rainfall and lessened evaporation. Called pluvial periods, they were characterized by the spread of vegetation and the formation of many lakes. Heavy precipitation in the West was responsible for two great lakes—Lake Lahontan of Nevada and Lake Bonneville of Utah (which today forms the Great Salt and Utah lakes). During the Pleistocene, volcanic activity and warping of the earth's surface occurred on the Pacific coast. The cutting of the Grand Canyon took place chiefly in Pleistocene time.

Fauna of the Pleistocene

Among the characteristic Pleistocene mammals of North America were at least four species of elephants, including the mastodon and the mammoth, true horses, of the same genus as the domestic horse though not of the same species, saber-tooth carnivores, large wolves, giant armadillos and ground sloths, bisons, camels, and wild pigs. Among the arctic mammals that ranged far south in the glacial stages were the musk ox in North America and the woolly mammoth in Europe. The Pleistocene saw the beginning of the trend toward the extinction of many mammal species, which continued into historic times. The Pleistocene is noted also for the first appearance of modern humans approximately 500,000 years ago and the migration of humans to the American continents.

Paleocene epoch, first epoch of the Tertiary period in the Cenozoic era of geologic time (see geologic timescale) between 60 to 66 million years ago. In W North America, the uplift of the Rocky Mts. that marked the end of the Mesozoic era continued throughout the Paleocene, and the Cretaceous inland seas gradually withdrew from the Great Plains area and central and SW California. In Montana and Wyoming the Fort Union shales and sandstones, laid down during this epoch, are noteworthy because they overlie undeformed upper Cretaceous sediments, thus recording the demise of the dinosaurs and the rise of mammals. The Paleocene mammals were mostly small herbivores similar to their Mesozoic ancestors. By mid-Paleocene, the ungulates, or hoofed mammals of mostly five-toed forms, became abundant. Prosimian primates (tree shrews and tarsiers) also increased in number. Except for part of N France, Europe was largely emergent (i.e., above water). During this epoch, the opening of the Norwegian Greenland Sea eventually resulted in a much more significant mixing of waters, creating the cold North Atlantic Deep waters. Greenland began separating from Europe as the northern mid-Atlantic Ridge formed. On the other side of the world, Antarctica and Australia had separated; India had completed its separation with Africa, resulting in an outpouring of basalts; and India, Africa, and Australia were about to collide with Eurasia. By the end of Paleocene time, N America's last large sea retreated to the Gulf of Mexico. Some of the fossil evidence from Paleocene sediments is difficult to explain; Alaska, for example, clearly had broad-leafed evergreen floras that typically grow in tropical forests. As the land has not changed significantly in latitude since the Paleocene, the evidence of these floras is a puzzle.

Oligocene epoch, third epoch of the Tertiary period in the Cenozoic era of geologic time, lasting from 38 to 24 million years ago. More of North America was dry land during the Oligocene than in the preceding Eocene epoch. The Gulf Coast was flooded, but the Atlantic coast N of South Carolina became emergent; the principal formation of the Gulf district was the Vicksburg limestone. The Pacific coast, like the more northern Atlantic coast, was largely elevated; erosion led to the deposition of the Oligocene portion of three sediments (the Sespe conglomerate, sandstone, and shale), which contains red beds like those of the Permian period. The great erosion of the Rockies was responsible for the deposition of the fossil-rich White River clays and sands over large areas of W Nebraska and NE Colorado and parts of Wyoming and the Dakotas. Late in the Oligocene, the John Day deposits of volcanic ash, notable for their included fossils, were formed in Oregon. In S Europe, the formations are somewhat similar to those of the Eocene; a sandstone and shale formation, the Flysch, was laid down in regions adjacent to mountain systems. The Alpine mountain building episode reached peak intensity as Africa further impinged against the Eurasian plate (see plate tectonics). In the middle Oligocene a sea extended over N Europe as far east as the Urals and was connected with the greater Mediterranean through the present Rhine valley. There are extensive deposits of Oligocene lignite in Germany, indicating swamp conditions either before or during the flood, and the Alsatian potash, salt, and gypsum are Oligocene. During the Oligocene there was considerable volcanic activity in central Europe, Scotland, Ireland, and Iceland, as well as in the San Juan Mts. of Colorado and the Absaroka Mts. of Wyoming where remnants of this volcanism persist in Yellowstone National Park. The life of the Oligocene was marked in Europe and North America by the virtual disappearance of the archaic mammals of the Paleocene. Carnivorous mammals—ancestral dogs and cats—made their appearance, along with beavers, mice, rabbits, and squirrels. A more highly developed type of horse, giant hogs, and camels were other new arrivals. The titanotheres—mammals remotely related to the horse and the rhinoceros—evolved to types of great size, then died out. The brontotherium, which appeared in North America, was the largest mammal to ever live on that continent. Running and aquatic rhinoceroses developed. The earliest elephant and a primitive anthropoid ape appeared in Africa. The climate of the Oligocene was mild and temperate in North America.

Miocene epoch, fourth epoch of the Tertiary period in the Cenozoic era of geologic time (see Geologic Timescale, table), lasting from around 24.6 to 5.1 million years ago.

North America was more extensively submerged in the Miocene than in the preceding Oligocene epoch and underwent considerable crustal disturbances. The Atlantic and Gulf coasts were flooded about as extensively as in the Eocene epoch. Miocene rocks are found along the Atlantic as far N as Martha's Vineyard, but the series, everywhere thin, is thickest and least interrupted from New Jersey to Maryland. On the Gulf coast it extends from Florida westward to Texas. The Atlantic series is chiefly marls, clays, and sands, with diatomaceous earth; the Florida series, chiefly limestone (Florida having risen as an island in the late Oligocene); the Gulf series, limestone and clastic sediments.

On the Pacific coast, the Great Valley of California was submerged at the beginning of the Miocene. The deposition of the Vaqueros sandstone, clay, and conglomerate was followed by the formation of the oil-rich Monterey series, partly sandstone and shale but largely diatomaceous tufa. In mid-Miocene time there was extensive mountain building in this region; the Cascades and Coast Ranges were elevating, although the Rocky Mts. had by then eroded to low relief. This disturbance was accompanied by volcanic activity—the Columbia and Snake river plateaus consist of over 200,000 sq mi (520,000 sq km) of basaltic lava flows up to 10,000 ft (3,000 m) thick—and by the first known movement along the San Andreas fault zone, engendered by the collision of the North American continental plate with the Pacific Ocean plate (see plate tectonics).

Late in the Miocene a new, extensive submergence resulted in the deposition of the San Pablo shale and sandstone. The sediments of the California Miocene came chiefly from the Sierra Nevada and the Klamaths, which, through erosion, were peneplained by the close of the epoch. In the western interior of North America the Columbia River basalt plateau of Idaho, Washington, Oregon, N California, and N Nevada was formed by a great outpouring of lava, which continued in the succeeding Pliocene epoch.

During the Miocene most of N Europe was elevated, but marine waters covered E Spain, S France, Italy, and a depressed area extending through Hungary to a basin around Vienna. In addition to considerable mountain making, lagoons were formed at the base of the Carpathians and north of the Caucasus in the regions now occupied by the Romanian and Baku oil fields.

The mammalian life of the Miocene was marked by further stages in the development of the horse, by the multiplication and final extinction of the giant hogs, and by the appearance of the mastodons, raccoons, and weasels. Cats, camels, doglike carnivores, and rhinoceroses were common, and species of a great ape (Dryopithecus) inhabited S Europe, Asia, and Africa. In the Miocene a distinct cooling of the climate resulted in the reduction of forests and an increase in grassy plains.

Holocene epoch or Recent epoch, most recent of all subdivisions of geologic time, ranging from the present back to the time (c.11,000 years ago) of almost complete withdrawal of the glaciers of the preceding Pleistocene epoch. During the Holocene epoch, the sculpturing of the earth's surface to its present form was completed. Withdrawal of the glacial ice resulted in the development of the present-day drainage basins of the Missouri and Ohio rivers, the development of the Great Lakes, and a global rise in sea level of up to 100 ft (30 m) as the glacial meltwater was returned to the seas. Warming climates resulted in the poleward migration of plants and animals. The most significant development during the Holocene was the rise of modern humans, who are thought to have first appeared in the late Pleistocene. All of the races of modern humans were fully developed, with eventual worldwide distribution. Human culture developed during this epoch from a primitive one to the complex industrial society of today, in which humans themselves have become a significant factor in altering the earth's surface environment. See Geologic Timescale (table).

Eocene epoch, second epoch of the Tertiary period in the Cenozoic era of geologic time, from approximately 54.9 to 38 million years ago. The Eocene in North America was marked by the submergence of the Great Valley of California and a portion of the Atlantic and Gulf coastal plain extending from New Jersey to Texas and into the present Mississippi River valley as far north as S Illinois. There was also extensive sediment deposition in the Rocky Mt. region. Eocene sedimentary formations along the Atlantic-Gulf coast are chiefly sands, clays, and marls, with some limestone and lignite; in California, Oregon, and Washington they consist of shale and sandstone, with oil and coal. The Badlands of the West are partly cut into Eocene rock formations, e.g., the Wasatch, Green River, Bridger, and Uinta formations, which contain great quantities of volcanic ash and, in some districts, oil-producing shale. The Green River formation of SW Wyoming is noted for its freshwater fossil fish. The brightly colored Wasatch formation makes up the spectacular pillars of Bryce Canyon National Park. Interpretation of Eocene rock strata is based on the succession of beds in Belgian, French, and English basins, which became type areas. The Norwegian-Greenland Sea began to open during the Eocene, and a great inundation from the Mediterranean covered most of S Europe, N Africa, and SW Asia, depositing nummulitic limestone, which is prominent in the Alps and Carpathians and from which the stones of the Pyramids were quarried. Mammals became the dominant animals, and the ancestors of the common animals of Europe and North America made their appearance, possibly as immigrants from other regions. Eocene mammals included ancestral rhinoceroses, tapirs, camels, pigs, rodents, monkeys, whales, and the ancestral horse, eohippus, as well as animals such as the titanothere, which have since become extinct. The vegetation of the Eocene was fairly modern; the climate was warm.

or glacial age

Any geologic period during which thick ice sheets cover vast areas of land. Such periods of large-scale glaciation may last several million years and drastically reshape surface features of entire continents. A number of major ice ages have occurred throughout the Earth's history; the most recent periods were during the Pleistocene Epoch (1.8 million–10,000 years ago).

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Unit of geologic time during which a rock series is deposited. It is a subdivision of a geologic period. Additional distinctions can be made by adding relative time terms, such as early, middle, and late. The use of the term is usually restricted to divisions of the Tertiary and Quaternary periods.

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